1. Field of the Invention
The present invention relates to an onion-like carbon thin film and its production method. Onion-like carbon thin films of the present invention are very useful in terms of availability in various industrial fields as hard protective films and solid lubricating films of the surfaces in various machine parts, electronic parts, and the like, or as field electron emission materials, and the like, electronic parts of a field emission display, and the like.
2. Description of the Prior Art
As the crystallized form of carbon, two types, i.e., diamond and graphite are known in the art. However, C60 (commonly called fullerene) was discovered as a novel soccer-ball-like crystallized form by Kroto et al., in 1985 [see H. W. Kroto et al., xe2x80x9cNaturexe2x80x9d, 318 (1985), 162]. By subsequent studies, fullerene molecules, higher fullerenes, solid-phase fullerenes, fullerene compounds, metallofullerenes, nanotubes, nanocapsules, and onion-like carbons were found as fullerene substances. These fullerene substances exhibit diverse characteristics such as superconductivity, conductivity, semiconducting property, insulating property, ferromagnetism, and super-atomic property. Accordingly, they have possibilities of being applicable to various uses such as nanotransistors/diodes, electron emissive elements, single-electron devices, photosensors, electrophotographic photoreceptors, very high speed electronic devices, superconductive magnets, solar batteries, solid lubricants, three-dimensional non-linear optical elements, very high speed optical switching elements, light-weight permanent magnets, gas storage units (oxygen, hydrogen, argon, and the like), and super-atomic structures.
Out of these, an onion-like carbon is referred to as various names such as bucky onion, onion-like fullerene, and onion-like graphite, and it is a kind of giant fullerene. It has a structure in which around the perimeter of the core at the central portion comprising C60 or the like, fullerenes each having a still larger molecular weight are concentrically stacked one on another. The onion-like carbon having such a giant fullerene structure is expected to find application as various functional materials centering on the electronic devices as described above not only because it has physical properties resulting from its specific structure but also because novel functions are exhibited by trapping metal atoms [ex., Pd, and the like, (Oku et al., xe2x80x9cMaterials Integrationxe2x80x9d, 12 (1999), 97)], hydrogen atoms, or the like therein.
For synthesis of fullerene substances, there are proposed a very high temperature synthesis method by an arc discharge method, or the like [see, ex., T. W. Ebbesen et al., xe2x80x9cNaturexe2x80x9d 358 (1992), 220], a method in which a mixed solution of a phenol resin and a cellulose derivative is baked (JP-A-No. 1311/1999), a laser ablation method in which an ultraviolet laser is applied to the graphite surface, and the like. However, since the fullerene substance is generally extracted from a soot-like substance, it is considered difficult to form it in thin film. For this reason, there is proposed a method in which a thin film is formed by using a specific fullerene derivative with an LB (Langmuir Blodgett) method (JP-A-No. 322682/1999). As the methods for manufacturing a carbon fiber material having a cross section of an onion-like structure, there are proposed a method in which a meso-phase pitch is subjected to melt-spinning, and then subjected to an infusibilization treatment and a baking treatment to form a carbon fiber (JP-A-No. 212626/1998), a method in which a carbon fiber and a carbonaceous binder are formed in lamination, followed by baking at a temperature of 2300xc2x0 C. or more (JP-A-No. 125334/1998), and the like. Further, for the onion-like structure, there is known a method in which an electron beam is applied to an amorphous carbon. For example, there are proposed a method in which ultra-fine particles forming nuclei are disposed on an amorphous carbon substrate, and then irradiated with a high energy beam (JP-A-No. 92124/1999), a method in which a substance having a triple bond or a double bond of carbon is irradiated with X-rays, a microwave, a supersonic wave, electron beams, and the like (JP-A-Nos. 109310/2000 and 16806/2000), and the like.
However, the foregoing methods have the following problems.
It is difficult that the soot-like materials and fibrous materials obtained by, for example, the methods disclosed in JP-A-Nos. 1311/1999, 212626/1998, and 125334/1998 described above are applied and developed to be adaptable for use as electronic devices in terms of their forms. This is because a thin film-like material having an appropriate thickness cannot be obtained on a substrate such as a silicon wafer as with an LSI or the like by the foregoing methods.
Further, with the synthesis methods by electron beam irradiation disclosed in JP-A-Nos. 92124/1999, 109310/2000, and 16806/2000, the probability that onion-like structures are homogeneously formed on the irradiated substrate is low. Therefore, it is difficult to form such a compact thin film that the film is filled up with onion-like structures. Further, there is also a problem that a uniform film cannot be formed because of a difference in the manner of growth. Such being the case, it is impossible to obtain a uniform and compact thin film having onion-like structures over a wide range. On the other hand, with a substrate for a recent electronic device, a thin film with a relatively large film thickness [at least about 100 nm or more, and a thick film of xcexcm order (about 1-5 xcexcm) is also required when the film is patterned in a three-dimensional structure] becomes necessary with upsizing of the display area. In addition, there is a growing demand for a uniform thin film in which the cluster structures given thereto are made compact for a purpose of ensuring quality.
The present invention has been made in view of the foregoing circumstances. It is therefore an object of the present invention to provide an onion-like carbon thin film comprised of giant fullerene clusters having onion-like structures, and its production method.
The onion-like carbon thin film of the present invention which has accomplished the foregoing object is characterized by comprising carbon as a main component, having a film thickness of at least 20 nm or more, and having clusters of an onion-like structure. Specifically, the onion-like carbon thin film of the present invention further satisfies the following requirements (1) and (2).
(1) At least 20 or more clusters each having a diameter of 4 nm or more, and having an onion-like structure are contained therein per 0.001 xcexcm2; and
(2) The proportion of clusters each having an onion-like structure in a matrix is at least 50% by volume or more.
A method for producing the onion-like carbon thin film in accordance with the present invention, whereby the foregoing problems can be solved, comprising a step of sputter-forming a film to a film thickness of at least 20 nm or more such that the plasma space potential is lower than the surface potential of a base material to be subjected to film formation. Herein, it is the preferred embodiment of the present invention that the film is sputter-formed by applying the base material to be subjected to film formation with a positive bias voltage. Particularly, if the film is sputter-formed by using an unbalanced magnetron sputtering method, a desired onion-like carbon thin film can be obtained with efficiency.